In the "Chemically Active Fluid Bed" gasification process (CAFB) petroleum oil feed stock is converted into a hot combustible fuel gas. The hot fuel gas contains sulfur, predominantly in the form of hydrogen sulfide. In order to prevent environmental pollution by the combustion products of this gas, the sulfur content of the gas must be reduced to acceptable levels by desulfurization either before or after combustion, i.e., either by removing the H.sub.2 S from the fuel or by removing SO.sub.2 from the combustion gas.
British Pat. No. 1,183,937 describes a typical process for the removal of sulfur contaminates from such a gas. It describes a process in which a hot fluidized bed of calcium oxide (generated from limestone heated in situ) is employed to absorb the sulfur contaminants as calcium sulfide. The process may be operated regeneratively or nonregeneratively. In the regenerative embodiment described in the British patent, the fluidized bed material is regenerated by heating to about 1000.degree.C in a mildly reducing atmosphere to remove sulfur oxides and the regenerated calcium oxide is returned to the absorption vessel. Similar processes are described in U.S. Pat. Nos. 3,402,998; 3,599,610; 3,625,164; 3,707,462; 3,717,700; and 3,781,408.
Even in those processes in which the lime is regenerated and reused in the desulfurization step, it is necessary to continuously or periodically remove a portion of the lime reactant and replace that portion with fresh lime (usually added in the form of limestone, which calcined in situ) in order to maintain the desired level of reagent activity. A typical spent lime material from such a desulfurization process will contain approximately 95% by weight CaO, 4% by weight CaS and CaSO.sub.4 and 0.8% by weight Vanadium. The spent lime removed from the system will react vigorously with carbon dioxide and water to release heat and hydrogen sulfide, rendering it a fire hazard and threat to animal life. Consequently, further treatment of the spent lime is necessary in order to render it environmentally acceptable and safe for general disposal.
Several proposals have been set forth for the recovery of by-products by the conversion of the sulfur dioxide in the regenerator exit gas to sulfuric acid or sulfur and for the treatment of the spent lime for conversion to some environmentally acceptable form. However, all of these suggested methods require two separate processes, one for treatment of the sulfur dioxide containing gas and a separate treatment for the spent lime. British Pat. No. 1,183,937 (page 2, lines 89-111), U.S. Pat. No. 3,707,462 (col. 3, lines 1-4), and U.S. Pat. No. 3,717,700 (col. 5, lines 63 to col. 6, line 7), all suggest the catalytic conversion of the sulfur dioxide from the regenerator to some useful by-product.
A process for the removal of sulfur values from a flue gas, and subsequent conversion to a construction material, is disclosed by Lin in U.S. Pat. No. 3,781,408. Lin passes flue gases from a combustion chamber containing sulfur dioxide through a catalytic oxidation converter to convert the sulfur dioxide to sulfur trioxide. Sulfur trioxide is then passed through an absorber having staged fluidized beds of lime to form calcium sulfate. The result is a mixture of calcium sulfate and calcium oxide which may be blended with fly ash and used as a construction material. The Lin process does not produce an environmentally stable product and its economic hinge on the market for the construction material which it produces.
U.S. Pat. No. 3,402,998, issued to Squires, describes a process wherein a hot fuel gas is reacted with calcined dolomite to absorb the hydrogen sulfide contained therein. The sulfurized calcined dolomite is then reacted with steam and carbon dioxide to produce a hydrogen sulfide-rich overhead which is treated in a separate process for the recovery of sulfur by-products. Squires teaches that the solids from the carbonization step may be calcined and reused in the fuel-desulfurization step.